On Carbon
Carbon is a big subject these days. Especially in environmental/global climate change circles. Carbon footprints, carbon credit, carbon debit… It would be nice if somebody explained exactly what carbon was, right?
So, here’s the short answer: in environmental-themed conversations, “carbon” refers to various carbon wastes and biproducts, most notably carbon dioxide and carbon monoxide. CO2 (or carbon dioxide), generally in units of mass or weight, is usually used as the “meterstick” – carbon emissions are measured in “tons of CO2 equivalent,” a number found by taking the tons of each type of emission, multiplying it by some factor relating its impact to that of pure CO2, and adding the results together.
(this is about where the short answer ends).
Doing this usually makes for a nice big number, one much larger than the total amount (in tons) of pollution emitted by a given plant. Why is that? you may ask. It is, I would reply, because pound-for-pound, carbon dioxide is one of the most innocuous chemicals, especially carbon-based “pollutants,” out there.
I will return to the subject of pollution shortly. In order to go more in-depth, I will have to explain a little about carbon’s chemistry. Please bear with me, as I will try to make this as non-painful as possible.
Carbon Chemistry
More important than anything else, “carbon” is an element. If I use the word “carbon” by itself below, I will be talking about the element Carbon (6) (as opposed to it being used with another word, like “monoxide”).
It is the sixth element in the Periodic Table. This may not tell you all that much if you’re not a chemist, but to those of us who are, it makes it one of the most interesting elements on the entire table.
You know that atoms are made of protons and neutrons in a central bundle called the “nucleus,” and even smaller “electrons” in a kind of cloud around it (they do not have fixed orbits around the nucleus like planets do around the sun, no matter what your grade school science teacher said; but if it helps you to think of them that way, it will not matter right now). Carbon has six protons in the nucleus, so the nucleus has a +6 (positive-six) charge. As a free element, Carbon would have six electrons; two of them in an “inner” orbital, and four more in “outer” orbitals. Each electron has a charge of -1, so the total charge of a free carbon atom would be 6 – 6 = 0, or neutral. The protons and electrons balance out in nature, and you are left with no charge, which is good (tangent: at this level of chemistry, I am always struck by how elegant this is – ).
But that’s not all there is to it, of course.
You see, as much as atoms like to be uncharged, they like to have full outer orbitals, too (the inner ones fill up before electrons start going into the outer ones). There are three ways for an atom to go about doing this:
- “give away” electrons until your outer orbital is empty, leaving the inner one (which is already full) – results in a positive charge (this process is known as “oxidation”[1])
- “Steal” electrons until your outer orbital is full, resulting in a negative charge (known as “reduction”)
- “Share” electrons with another atom or atoms, on a 1:1 basis, until the total number of electrons flying around is equal to the number needed to fill your outer orbital (8, in the case of carbon) (this process is known as “covalent bonding”)[2]
- You get energy by oxidizing the carbon, increasing its formal oxidation state.
- Coal has an oxidation state of 0
- Ethanol has an oxidation state of -1
- Natural gas has an oxidation state of -4
- Most petroleum products have an average oxidation state of ~=-2.
- A GHG, in sufficient quantities, can lead to the “greenhouse effect” and “global climate change” of an unnaturally-fast sort
- A “pollutant” causes direct or nearly-direct environmental harm, through such mechanisms as killing life, promoting too much of one kind of life, or in heavy-metal-poisoning building-up-through-food-chain methods
Note that all of these involve only electrons. In chemistry, it’s all about electrons, because they’re the things that are in the outer clouds, the things that can be easily picked up and discarded. You cannot balance charges or fill orbitals by gaining or discarding protons – for one, they’re held together in the nucleus pretty tightly, and for two, if you do, you make a really, really big explosion (think Manhattan Project).
In general, the first option is used by atoms that have outer orbitals which are less than half-full (a description which includes all of the “metallic” elements). The second option is used by many non-metallic elements, those with more than half of their orbitals filled, especially those only requiring one or two more electrons. However, the more electrons you add (or remove) to an atom, the harder it is to add (or remove) more – because the atom has become charged, and you are trying to make it more charged[3]. This is not easy (think magnets; think how opposites attract, but like repel; it’s kind of like that).
Carbon is right in the middle. If you coaxed it into giving up four electrons, you’d have a +4 ion with a complete outer shell[4]; if you convince it to accept 4, you have a -4 ion. Either one is really hard to do. Really hard. Too hard. Also, I did not mention this before, but high-level ions (when that number gets higher than about 1 or 2) aren’t all that stable, and tend to chemically react to something more stable. And when you do have an ion, it has to be closely balanced by another ion – a +1 with a -1 (in Na Cl, salt), a +2 with a -2 (in MgO, magnesium oxide), a +2 with two -1′s (CaCl2, calcium chloride), or something else like that. Sometimes this is ok or the “best thing to do,” but often it isn’t[5]. It is really not OK when you have a right-in-the-middle-of-the-road element like Carbon, which is why Carbon, when incorporated into a molecule, does so exclusively through covalent bonds, and does not carry charges[6].
So, in summary: in order to fulfill itself, carbon atoms form a total of four covalent bonds with other atoms.
Yay! That’s it for… wait… no?
No, we are not done with chemistry yet.
You see, the atoms that carbon bonds to – and the number of bonds it forms with each atom – are important, too. I’m trying to think of the best way to explain it.
Let me start by creating a hypothetical system, in which there is only carbon. You can have carbons single-bonded to each other (each carbon bonded to four other carbons), carbons double-bonded to each other, or carbons triple-bonded to each other (which means each one is also single-bonded to another). You might thing that two carbons quadruple-bonded to each other would be the simplest solution to all problems, but alas, this is not possible (see the Wikipedia article on orbital hybridization, especially the section on sp hybridization, or just take my word that the geometry does not work out, anyway).
Speaking of geometry, a molecule made of entirely single-bonded carbon wouldn’t work out either. That leaves all-double-bonded, alternating triple-single bonds, and mixed double-single. You might think a long line of double-bonded carbons would work, but for some reason this is never observed; same with the alternating triple-single. The three forms that carbon takes on all involve alternating, 3d patterns of double and single bonds (these forms being graphite/coal, diamond, and fullerenes).
This is a bit too abstract, and it doesn’t matter all that much anyway, not for what I’m trying to teach. Let’s envision another system, in which in addition to Carbon, you have the elements Hydrogen (element 1, likes to share or donate a single electron) and Oxygen (element 8, likes to “steal” or share two electrons). You should probably just take my word that, in organic-type chemistry at least (the kind we’re concerned about here, since we’re talking about carbon, and eventually the environment), most other atoms you’ll find are analogous to one of these three. And besides, a really vast majority of organic molecules are made from these three (sugars, other carbohydrates, cellulose, fats, oils, lipids, waxes, water…). And with that, I’d like to introduce the concept of electronegativity.
Electronegativity
“Electronegativity,” roughly translated, means “how strongly a given atom pulls electrons that may or may not be on another atom.” It is a relative measurement, meaning one element (Fluorine) is given an arbitrary value (4.0), and other atoms are compared to it (Fluorine happens to be the most electronegative atom, so this works out really well). Hydrogen’s electronegativity is 2.2; Carbon’s is 2.55; Oxygen’s is 3.44. So, you see, carbon can “pull” electrons away from hydrogen, and oxygen can “pull” them away from either of the others. So when you have an oxygen bonded to a hydrogen, the two electrons involved in the bond spend more time around oxygen than around hydrogen. This isn’t the same as actually transferring them, since you don’t have a fully negative oxygen and positive hydrogen; but you kind of have partial charges on both of them. Likewise, a carbon has a partial positive charge when bonded to an oxygen (which would be partially negative), and hydrogen has a slight partial positive charge when bonded to a carbon atom (which would have a slight partial negative charge) (these partial charges would be much smaller than H-O or even C-O, but they exist nonetheless).
Got it? Good. You are now ready to go on to
Formal Oxidation States
Take a bond. Say a hydrogen bonded to a carbon. Say, “what would happen if I took all of these partial charges, and made them full charges?” What would you have? A +1 H, and a -1 C. These are the Formal Oxidation States of the atoms in these bonds. The real formal oxidation state involves adding up the total charges for all the bonds an atom has, which is easy enough for hydrogen (which only has one bond) or oxygen (two bonds), but is a bit more complicated for carbon, which has four. Take this example:
O
||
C
/ \
H H
The central atom is carbon (we already know the other oxidation states, anyway; -2 for the O, +1 for each of the H’s). A double-bond to oxygen (that’s the ||) means four electrons shared, is the same as two single-bonds to different oxygens, each of which is a -1 for the O and a +1 for the C. So, +2. Now below – the H’s. Each one is a -1 for the C, and there’s two of them, so that’s -2. So the carbon’s formal oxidation state is -2 + 2 = 0 – neutral! How cool is that?
The formal oxidation state of the atom is worth taking a look at too: -2 + 1 + 1 + 0 = 0. The molecule is uncharged, as expected (if you’re having a hard time finding the oxidation state of an atom, and know the total charge of the molecule, try doing this, but in reverse – use that algebra for something!)
I brought up formal oxidation states for a reason, a reason to which I intend to arrive soon. Soon being now.
“Burning” is a crude term for “performing oxidation upon (using oxygen as the oxidant).” Burning things is how we get most of our energy, and almost all of what we burn is carbon based. Now, the extreme ends of Carbon’s oxidation states are -4 and +4. And there is a range in-between. But the general idea is, you take carbon of a low oxidation state (the lowest of which is -4) where it has lots of electrons, and start transferring those electrons to a more electronegative thing – oxygen – until there are no more to transfer (oxidation state +4). Each time the carbon “loses” a formal electron/gains a formal oxidation state, it releases energy, and increases entropy. You know this process quite well.
Now, quiz time. Given the three atoms we’re working with, what are the two extremes of oxidation states?
Answer: Methane (CH4) at -4; and carbon dioxide (CO2 or O=C=O) at +4.
And… I’d like to illustrate example in-between, which I will now proceed to do, since I can’t think of a transition. (In cases where there’s more than one C, the bold one is the one I’m looking at. Also, I forgot to mention before, but a C-C bond has no effect on the formal oxidation state of either C).
| Molecule | O=C=O | O=C-OH-CH3 | O=CH-OH | O=CH-(CH3)2 | O=CH2[8] |
|---|---|---|---|---|---|
| Formal Oxidation State | +4 | +3 | +2 | +1 | 0 |
| Molecule Name (if any) | Carbon Dioxide | Formaldehyde |
| Molecule | HO-CH2-CH3 | HO-CH3 | H3C-CH3 | CH4 |
|---|---|---|---|---|
| Formal Oxidation State | -1 | -2 | -3 | -4 |
| Molecule Name (if any) | Ethanol | Methanol | Ethane | Methane (main component of natural gas) |
The +1, +2, and +3 ones are kind of weird, but it’s important to understand, mostly, that they exist. Coal is equivalent to the 0 state (as are wood and sugar[9]), and natural gas, being mostly methane, is equivalent to the -4 state. So, to reiterate.
Hold on to this knowledge. I will be referring to it soon.
I believe I am done with Formal Oxidation States, and carbon chemistry in general (questions or clarifications, ask in Comments).
Carbon – pollution
When talking about carbon as a pollutant, people are normally referring to Carbon Dioxide and its effects as a “greenhouse gas.” For the remainder of this essay, I would like to separate (except where noted) the definitions of “pollutant” and “GHG”
Thus, by the definitions I am using, carbon dioxide in moderate quantities (the kind humanity puts out today) is a GHG but not a pollutant. This might not be your definition, and if you would like to quibble the point, please write up a nice long letter, and email it to /dev/null.
But let’s go with just the GHG’s for now.
Say you had to choose a fossil fuel. This fuel would be completely combusted, and everything except final combustion products (i.e. water and CO2) is completely removed and disposed of properly. You want to maximize energy produced per unit CO2 produced. Which do you choose?
You could choose coal. You would travel 4 oxidation states to CO2, and produced a decent amount of heat.
You could also choose fuel oil, travelling 6 oxidation states to CO2, thus producing 2 oxidation state’s worth more heat. This seems like a better choice to me.
OR you could choose natural gas, travel 8 oxidation states to CO2, and produce a lot of heat (therefore energy). It is your best choice, and you should go with that.
Yay! We’re learning. Natural gas is less-GHG-bad than coal or gasoline.
Now on to pollution for a few moments.
As a pollutant, carbon dioxide is very innocuous. You have to have rather high concentrations (compared to current atmospheric concentrations) to kill anything. You wouldn’t kill everything, anyway – to plants, CO2 is an input to photosynthesis, not just an output from digestion.
Formaldehyde (O=CH2) is pretty deadly stuff. It used to be a common preservative for dead animals etc, until they found out how carcinogenic it is.
Methanol and ethanol are moderately bad. Similar chemicals are much worse.
Methane and ethane kind of evaporate too quickly to cause much harm.
There are many more carbon-based pollutants out there. Out of all of them, CO2 is probably the least harmful when inhaled/ingested.
CO2 is relatively harmless as a GHG, too (everything hereinafter references the Wikipedia article on Greenhouse Gas, I am too tired/lazy to link / cue flame-bombs from environmentalists! “You trust Wikipedia? Don’t you know it’s edited by fascist corporate slaves who only want to kill the trees and make a few bucks? HOW COULD YOU??” – I apologize to any hippies I have just offended). Methane is also a relatively-commonly emitted pollutant (think cows… think digestion). As a greenhouse gas, it has 72 times the effect of CO2 over 20 years; due to spontaneous reaction with oxygen, this number goes down to 5.6 times over 500 years. But this is still significant (also because, when it degenerates, it forms CO2). But if you burn it first, you have CO2 for that whole time – that much less contribution! Wow!
There are other contributors, like nitrous oxide, CFC’s, HFC’s, and carbon tetrafluoride. The gist is, they are all many many times more harmful than CO2 on a per-mass scale. So, why the big deal about CO2?
Because we make much more CO2 than anything else. Orders of magnitude, likely.
It’s a quality vs. quantity thing. Sure, carbon tetrafluoride has over 11,000 times the effect of CO2 over 500 years, and sure it’ll be around for a lot longer; but carbon tet is a rather well-controlled substance, and the amount released in a year is (as a rough guess) a couple tons; CO2, however, is released in thousands and thousands of thousands of tons per day, its quantity vastly outstripping the quality of other GHG contributors.
Well, not quite that much. There’s lots of sulfur in coal, see. And nitrogen is always a problem, it’s in the air, so when you’re burning something, it gets mixed in too… so say you make 1 ton of CO2, and 20 pounds of GHG number 2. GHG number 2 is 100 times as powerful a GHG as CO2. So, you’re releasing 20 * 100 = 2000 pounds CO2 equivalent of it – or 1 ton – or 2 tons total CO2 equivalent.
So, yeah. Inflated numbers. It bothers me.
Staying up late to finish blogs, then realizing the next day I could’ve written them better, bothers me as well. But until my Gentle Readers complain, I’ll keep doing it, ok?
I will go to bed now, during which time I’ll have dreams about being chased by molecules I haven’t seen or drawn in years, but which have resurfaced thanks to research for this post. I hope you’re happy now.
-Goodnight>
[1]“Oxidation” because Oxygen (the molecule) is very good at causing this to happen to other atoms, especially metals
[2]To explain a bit more, let me talk about hydrogen (which has one atom, and needs a total of two). Two hydrogen atoms can get together and share their single electrons with each other. Each atom donates one electron per bond, so each bond has two electrons. You can have more than one covalent bond between two atoms, and an atom can covalent bond with more than one atom; but the total number of covalent bonds cannot, in general, exceed the number of electrons in its outer orbital to begin with (so hydrogen, for example, can only form one, while carbon can form up to four). If you’d like to get a firmer grasp of the concept, look up some “Lewis structure exercises” or examples and practice with them a bit. Also, it gets more complicated when more than two atoms are involved in a single bond, but that is not something you need to worry about until Organic Chemistry, which we will not be doing here 
[3]Properly speaking, when an “atom” becomes charged, it should be called an “ion” rather than an atom. I’m going to stick with “atom” as a generic term for both atoms and ions for now, since it’s less confusing to you, ok?
[4]“Shell” is another word for “orbital.”
[5]Because of thermodynamics. Specifically, entropy. More specifically, because when you have ions like this, they tend to arrange themselves according to charge – each negative ion surrounded completely by positive ions, and vice-versa. This leads to the growth of highly-ordered structures called crystals, which you may have seen before. Increasing order means decreasing entropy (entropy being an abstract measure of “disorder”), and the Second Law of Thermodynamics tells us that, in any closed system, entropy must always increase. Forming crystals decreases entropy, which means you’d better be producing a helluva lotta entropy from pushing those electrons around in the first place.
[6]Carbocations and carbanions (molecules where there is a charge on carbon) do exist, but they are not permanent; they’re often transition states or extremely (nano or pico-second lifetime) short lived intermediates. So when I say “it does not carry a charge” I mean “It is possible to give it a charge, but it does not stay there for a measurable length of time, and therefore cannot be counted as having been “carried” – unless there’s resonance, which only partially counts anyway.” And I’m not even going to mention free radicals, because they’re just crazy (this applies equally to the political and chemical definitions of “radical”).
[7]Assuming the carbon was bonded to nothing else, which is of course ridiculous, but beside the point.
[8]Also: C bonded only to other C’s, as in the case of coal or diamonds.
[9]People who remember Bio 1 are saying “hah! I caught you! WHICH CARBON in sugar?” To which I reply “no specific one; just take the average of all six.”
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